Many different streams containing mixtures of hydrogen and light hydrocarbons, such as C.sub.1 -C.sub.6 hydrocarbons, are generated during oil refining and petrochemical manufacture.
Over the years, economic pressures have driven refiners to attempt to convert even the heaviest fraction of the crude oil to gasoline components, fuel oils and petrochemical feedstocks. For example, hydrocracking is widely used to break down aromatic cycle oils, coker distillates and other relatively heavy feeds and reconstitute them as diesel fuels, kerosene or naphtha. This process is a heavy consumer of hydrogen, using perhaps 1,000-2,000 scf/barrel of feedstock cracked, and yields streams from which it is very desirable to recover as much hydrogen as possible for reuse. Separation of the raw stream leaving the reactors is typically carried out by flashing off hydrogen, followed by various stripping and fractionation steps as appropriate. Nevertheless, considerable amounts of hydrogen are not recaptured and pass to the fuel gas line with unrecovered light hydrocarbons.
Likewise, demand for low-sulfur products has increased, and refineries are reaching or have reached the point at which they can consume more hydrogen in desulfurization and related hydrotreating than they can produce from catalytic reforming. For example, desulfurization of middle distillates typically consumes about 600 scf of hydrogen per barrel of treated feed; for vacuum gas oil this number rises to about 800 scf/bbl and for atmospheric residue to about 1,000 scf/bbl.
Other representative processes carried out in refineries or petrochemical plants that can give rise to streams containing some hydrogen include catalytic cracking, catalytic reforming, delayed coking, distillate dewaxing, aromatics production, alkylation, isomerization, hydrogenation and dehydrogenation, and olefin production. Hydrogen-containing streams also arise from unsaturated and saturated gas plants used to treat and fractionate pooled off-gases from the various unit operations.
These many sources give rise to diverse streams from which it is currently not cost effective to carry out further hydrogen recovery, and, in general, these gases are simply used as fuel within the plant. Yet these streams range in volume flow from less than 1 MMscfd up to 20 MMscfd or more, and contain from less than 1% hydrogen to more than 70% hydrogen. Furthermore, many streams also contain high percentages, such as 10%, 20%, 30% or more, of C.sub.3+ hydrocarbons. The chemical value of these individual components is much higher--in some instances, as much as eight times higher--than their fuel value. The ability to recover at least some of this value would be advantageous, especially in refineries, which generally operate at narrow financial margins.
Separation of certain gas mixtures by means of selective membranes has been known to be possible for many years, and membrane-based gas separation systems are emerging to challenge conventional separations technology in a number of areas. That membranes have the potential to separate organic vapors from other gases is also known. For example, U.S. Pat. Nos. 4,553,983; 4,857,078; 4,963,165; 4,906,256; 4,994,094; 5,032,148; 5,069,686; 5,127,926; 5,281,255 and 5,501,722 all describe membranes, systems or processes suitable for such separations. Likewise, it has been recognized that condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843 and 5,374,300.
The use of certain polymeric membranes to treat off-gas streams in refineries is described in the following papers: "Hydrogen Purification with Cellulose Acetate Membranes", by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; "Prism.TM.Separators Optimize Hydrocracker Hydrogen", by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; "Plant Uses Membrane Separation", by H. Yamashiro et al., in Hydrocarbon Processing, February 1985;and "Optimizing Hydrocracker Hydrogen" by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. These papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons. The use of membranes in refinery separations is also mentioned in "Hydrogen Technologies to Meet Refiners' Future Needs", by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons.
A chapter in "Polymeric Gas Separation Membranes", D. R. Paul et al. (Eds.) entitled "Commercial and Practical Aspects of Gas Separation Membranes", by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
Hydrogen recovery techniques including membrane separation for use in refinery operations are described in many patents. U.S. Pat. No. 4,362,613, to Monsanto, describes a process for treating the vapor phase from a high pressure separator in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the hydrocracker reactor. U.S. Pat. No. 4,367,135, also to Monsanto, describes a process in which effluent from a low pressure separator is treated to recover hydrogen using the same type of hydrogen-selective membrane. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane again permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L'Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from cracking effluent, the hydrogen separation being accomplished by a hydrogen-selective membrane. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and light hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
Other references that describe membrane-based separation of hydrogen from gas streams in a general way include 4,654,063, 4,836,833, to Air Products, and 4,892,564, to Cooley.
U.S. Pat. No. 5,332,424, to Air Products, describes fractionation of a gas stream containing light hydrocarbons and hydrogen using an "adsorbent membrane". The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,435,836; 5,447,559 and 5,507,856, which all relate to purification of streams from steam reformers. U.S. Pat. No. 5,634,354, to Air Products, discloses removal of hydrogen from hydrogen/olefin streams. In this case, the membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen.
U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
A reference that shows condensation to remove hydrocarbons upstream of a membrane separation step in a refinery is U.S. Pat. No. 5,452,581, to Dinh et al. Effluent from an ethylene manufacturing operation is cooled to a temperature below 0.degree. C., such as -30.degree. C. to -50.degree. C., before passing the remaining stream to a hydrogen-selective membrane. Interestingly, in this case, the membrane is specifically used to raise the dewpoint of the remaining stream to facilitate subsequent cryogenic condensation.